This invention relates to a method for manufacturing contact lenses which involves casting a polymerisable monomer composition within a mould formed by two plastic mould halves. In addition, the invention extends to a novel design of plastics moulds for casting lenses and a system of manufacturing a packaged lens using elements of the mould in which it has been formed.
A method of growing importance for the large-scale manufacture of contact lenses comprises casting lenses in closeable moulds formed from a pair of co-operating mould halves. One problem which as to be overcome when casting lenses from polymerisable monomers in a closed mould system is to provide some means for compensating for the shrinkage which inevitably occurs when the monomer composition polymerises. Typically monomer materials used in contact lens production undergo a volumetric shrinkage of between 10 and 20%. Failure to compensate for this shrinkage will result in unacceptably high wastage rates and/or poor quality products containing voids or bubbles.
Various methods have been proposed for overcoming the problem of shrinkage during casting of monomer compositions. These have included providing a reservoir of monomer material which it is hoped will flow into and fill the cavity formed on shrinkage, see, for example, U.K. Patent specification No. 2,006,091. The difficulty with this kind of solution however is that the monomer tends to gel more rapidly in the confined area through which it is intended that the excess monomer should flow to reach the mould cavity.
One commercially successful solution to the shrinkage problem is taught by T. H. Shepherd in U.K. Patent No. 1,575,694. According to the Sheperd invention, a flexible peripheral rim or lip is provided on one of the two mould halves (normally the male mould half). The mould halves are designed so that the of closure of the mould the flexible rim contacts the corresponding surface on the other mould half to define the periphery of the mould cavity. When shrinkage occurs during polymerisation of the monomers, the flexible rim or lip deforms (normally inwardly) so as to permit the two mould halves to approach each other slightly more closely than when in the initial closed position. This slight movement of the mould halves towards one another during the polymerisation stage provides sufficient reduction in volume to compensate for the shrinkage of the monomers on polymerisation.
In general, and for many polymeric materials, the Shepherd method is extremely effective and has been very successful commercially. There are, however, some disadvantages which primarily arise from the way in which the edge of the cast lenses are formed. Satisfactory moulding of the delicate flexible rim or lip portion of the Shepherd mould requires great care and, in use, as the two mould halves are brought together, the flexible rim is easily distorted. This can result in a poorly shaped edge which, in many instances, requires mechanical polishing to be ophthalmically acceptable. In high quality lens production, edge polishing is necessary unless a very high rejection rate can be tolerated. While polishing is possible with many hydrophilic materials in their dry state this involves not only additional processing steps but incurs the danger of lens damage and losses during the dry processing stage. Furthermore, some lens materials such as silicon rubbers and fluorocarbon polymers are not capable of being polished because they are not sufficiently hard.
In one of its aspects the present invention is concerned with an alternative approach to the problem of compensating for shrinkage without the need to provide a flexible rim or lip on one of the mould halves.
U.K. Patent Specification No. 2 048 758 discloses a polypropylene mould for contact lenses, in which male and female parts of the mould have circumferential mating surfaces in the form of a horizontal shoulder on each mould part. The shoulders are formed as an interference fit so as to seal closely together when the male part is weighted at a load of 2 to 3 pounds, with the objective of avoiding flash at the periphery of the finished lens. The use of mating shoulders in this manner requires the two parts of the mould to meet with great accuracy to avoid variation in lens thickness around the lens.